The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Substrate processing systems may be used to perform etching and/or other treatment of substrates such as semiconductor wafers. A substrate may be arranged on an electrostatic chuck (ESC), or on a pedestal attached to an ESC, in a processing chamber of the substrate processing system. The ESC may be biased with an RF signal, using RF voltages in the range from tens to thousands of volts and RF frequencies in the range from tens of kHz to hundreds of MHz. Since the ESC also acts as a workpiece holder, proper control of the ESC temperature is an important consideration to ensure repeatable process results.
One or more electric heaters may maintain the ESC's temperature within a desired range. The heaters may be integrated or coupled with the ESC. Electrical power to the heater(s) typically is obtained from line AC voltage via an appropriate control circuit to maintain the ESC within a desired temperature range. By way of example, the electric heater(s) may be powered by DC, line frequency (e.g., 50/60 Hz AC) or kHz range AC power. The heaters may be operated at the same time, or at different times, depending on process requirements, chamber conditions, and the like, to maintain a temperature profile of the process. Maintaining the temperature profile facilitates better uniformity and etch rates in substrate processing.
In this configuration, the DC/low frequency power needs to be coupled to the ESC assembly, which is also simultaneously subject to substantial levels of RF power either by stray coupling or by direct connection. To prevent an undesirable apparent RF short to ground, loss of RF power and high levels of signal interference, even damage via the electric heater power supply and/or control circuitry, RF isolation is required.
Separate filters have been designed to block the RF current path from the heaters to their power source on different tools depending on RF-rejection frequency requirements. Designing RF filters has been tedious because, among other things, it has been necessary to place the parasitic resonances carefully between the harmonic frequencies to avoid unintentional RF-current. In addition, RF-rejection requirements have changed repeatedly as etch-rate and uniformity requirements have changed. Etch rates and uniformity requirements continue to change, so RF-rejection requirements may be expected to undergo continued change as well.
As a result, in lieu of specific filters for specific requirements, a universal solution for RF isolation has been sought. One such solution has been to replace all the RF-filters by designing a broadband and high power RF isolator which filters frequencies by providing a capacitive rejection response.
U.S. Pat. No. 8,755,204 discloses an approach to providing RF isolation. The patent proposes to reduce secondary-to-core capacitive coupling by, for example, providing a shield between a secondary winding and a core of an isolation transformer. The patent also proposes to reduce primary-to-core capacitive coupling by, for example, providing a shield between a primary winding and the core.
FIG. 1 shows relevant portions of an isolation transformer implementation, according to the above-referenced patent, to provide high DC or AC line power to a load that is also coupled to one or more high frequency RF signals. In FIG. 1, the load is a heater for an RF coupled chuck in a plasma processing chamber. A source power signal in the form of AC line voltages and frequencies (e.g., 50 Hz or 60 Hz) is supplied via leads 202 and 204 to a rectifier/filter circuit 206 which converts the AC line input power signal to a quasi-DC power signal which may be subsequently filtered into smooth DC if desired.
The DC power signal output by rectifier circuit 206 is then supplied to a drive circuit 208, which converts the DC power signal received on leads 210 and 212 to an intermediate signal having an intermediate frequency, for example, in the range of about 10 kHz to about 1 MHz, or in the range of about 10 kHz to about a few hundred kHz, and or in the range of about 10 kHz to about 200 kHz. As a result, the intermediate frequency is intentionally higher than the AC line frequency of 50-60 Hz but lower than the RF frequency to be blocked (which tends to be in the multiple MHz range). Because the intermediate frequency is higher than the AC line frequency, it is possible to use a smaller isolation transformer 220.
The intermediate signal output by drive circuit 208 is then supplied to the primary winding 222 of isolation transformer 220. Primary winding 222 is shown wound around one segment of a core 224. Core 224 may be formed of manganese zinc (MnZn) or nickel zinc (NiZn) or another suitable high magnetic permeability material (e.g., mu in the 2000 range). An air gap 230 may be provided in core 224, in which case primary winding 222 may be wound to the sides of the air gap 230 instead of over air gap 230, in order to reduce dissipation in the winding.
Secondary winding 236, which is not directly coupled to primary winding 222 by conduction, is also wound around core 224. To reduce capacitive coupling between primary winding 222 and secondary winding 236 and provide a high degree of isolation, particularly for the higher frequency RF signals, secondary winding 236 may be positioned apart from primary winding 222. For example, secondary winding 236 may be positioned opposite primary winding 222 around core 224, as FIG. 1 shows.
In the approach just described, the core is machined in a U-shape and requires the following elements to decrease capacitive coupling of the secondary to core, and of the primary to the secondary: i) larger diameter secondary winding, possibly wound on a plastic cylinder—the inner diameter of which will be stuffed with ferrite; ii) primary and secondary windings placed physically apart from each other but still magnetically coupled through the same core.
There are some challenges with respect to this approach. First, the cost to machine a U-shaped ferrite block, large enough to separate the primary winding from the secondary winding, and plastic cylinders (with precise grooves on the outer surfaces and holes for cooling the ferrite inside) can be difficult from a system stand-point. Second, when the ferrite is stuffed into the plastic cylinder on which the secondary winding is wound, it can be challenging to design an efficient cooling mechanism.
It would be desirable to provide RF isolation that is more comprehensive, and that does not require specially tuned circuitry, or intricate and/or expensive assembly and/or manufacture.